With the change in 7th edition to using dice pools for spell casting. I took note of the spell familiar in the Chaos Space marines codex. it allows you to reroll failed spell tests. My interpretation is you can re-roll each dice that is not successful as each dice is now separate casting regardless of how many you use to try and cast a spell. This would of course both increase the risk of perils of the warp and increase your successful spell casting dice count making it harder or forcing your opponent to use more defense dice to try to deny it. Is this a correct interpretation as the spell familiar is supposed to increase your chance to succeed in spell casting?

What this means is that you reroll a failed psychic test (not dice). A psychic test consists of all the dice you rolled; therefore, to take advantage of the spell familiar, you reroll all the dice.

For instance, if you wanted to manifest a Warp Charge 3 power and roll 1, 2, 2, 3, 4, 6, that is a failed psychic test because you only scored 2 and needed 3. Thus, you'd reroll the whole test, not specific dice, as that's what the rule says to do.

Make sense?

Any rule that deals with the psychic tests will specify "psychic test" or "psychic dice" for their effect.

"Serious sport has nothing to do with fair play. It is bound up with hatred, jealously, boastfulness, disregard of all rules and sadistic pleasure in witnessing violence. In other words, it is war, minus the shooting." - George Orwell

that's what I was trying to figure out. as the other dice were failed attempts, but what your saying is if it succeeds then you get what you get with what dice are good instead of being able to re-roll just the failed ones when the test is already successful. That sounds better and makes sense. I was doing a practice game verses GK and he was talking more along the line of what your saying having to reroll them all rather than just the failed dice.

Lets hope that GW and or Adeptus gives us a final FAQ on that as this book was made for 6th edition and not 7th which of course has a whole new rule set for casting spells. For now I will play it just like you said as that seems to be able to dismiss any arguments on how the spell familiar works.

Yea, the rules as they stand work just fine and help you dismiss any crazy opponents that say otherwise. For instance, while the Runes of Witnessing add +2 to a Psychic Test (turning a 1 success roll into a 3 success roll), the Runic Weapons add 1 to Deny the Witch rolls (so needing a 5+ for each DtW dice at minimum).

Basically, the rules will specify whether you add/reroll the "test" or just "certain dice". In this case, it's "failed test" instead of "failed dice". Example: If you get 1 success on a 2 warp charge power, you must reroll all the dice because that's what constitutes a "failed psychic test". If you get 2 successes on a 2 warp charge power, you don't get to reroll anything because it's a "successful psychic test".

"Serious sport has nothing to do with fair play. It is bound up with hatred, jealously, boastfulness, disregard of all rules and sadistic pleasure in witnessing violence. In other words, it is war, minus the shooting." - George Orwell